Downhole mining tools which provide jet disintegration and excavation of a mineral deposit and collection of the resultant slurry are well known, as exemplified by U.S. Pat. No. 4,140,346, issued to Ronald Barthel on Feb. 20, 1979. Generally, this type of tool uses an eductor pump, also popularly known as a jet pump, to provide necessary lift to bring the slurry to the surface. Also, a cutting jet directed laterally comminutes and slurrifies the mineral deposit. The eductor evacuates the excavated chamber as it gathers the slurry, so that the cutting jet flows through a gas medium, in most cases the gas being air.
A severe limitation of the eductor lift system, however, is that there is a maximum depth at which the eductor provides lift. Typically, eductor systems are limited to a depth of approximately 600 feet. While the depth range of eductor systems may be extended as by compounding the eductor with auxiliary lift methods, this entails additional complexity and expense.
Another limitation of eductor systems is that the roof of the excavated chamber tends to collapse as the chamber horizontal diameter increases. This limits the ability of a single borehole rig to exploit the mineral deposit.
A further drawback to eductor systems is that for proper control of the cutting and lifting functions, separate pumps are required to vary the flow and pressure of fluid supplied. Furthermore, separate conduits serving the cutting jet supply, lift jet supply, and slurry return may also be required. Containing three conduits within a single borehole casing devotes excessive cross sectional area to conduit sidewalls, thereby reducing the effective conduit area, increasing the attendant pressure losses due to sidewall friction, and increases the cost of construction. The reduced conduit cross section also increases the chances of a jam by large, uncomminuted particles in the slurry return line.
Therefore, a tradeoff is made between desired control and reduced power requirements. Illustratively, Barthel uses only two conduits; however, his arrangement forgoes adjustment of flow for cutting or lift relative to flow for the other function.
Packers for sealing the annulus between concentric tubes are known in the mineral drilling arts. An example of a hydraulically actuated packer is U.S. Pat. No. 3,142,339, issued on Jul. 28, 1964 to C. C. Brown et al. Prior art packers are generally intended to provide a tight seal or a tight grip between tubes.
A national trend towards depletion of easily recovered shallow mineral deposits makes it desirable to adapt borehole mining to deep hole applications. In the face of the above cited limitations and costs, even as boreholes are drilled deeper, economic necessity makes it imperative to render each borehole rig more productive and less costly.